In recent years, PWM converters have been widely used with an object of transmitting and receiving electric power in both directions from an AC power source to a DC power source or from a DC power source to an AC power source.
PWM converters are also often used with an object of reducing a phase difference of voltage and current of an AC power source, i.e., for the purpose of improving a power factor, and suppressing current distortion of an AC power source, i.e., reducing higher harmonics of the power source.
A typical conventional PWM converter system will now be described with reference to FIGS. 14 to 17.
In FIG. 14, it is assumed that a voltage between a plus terminal and a minus terminal of a smoothing capacitor 60 is higher than the maximum value of a phase voltage of a three-phase AC power source 1. First of all, in a current instruction generator 7, a phase information value .theta. and amplitude instruction value ip of the three-phase AC current wave form to be supplied from the three-phase AC power source 1 are set, and on the basis of these values of information, the current instruction generator 7 generates respective line current instructions that are to be input from the three-phase AC power source 1: these are first line current instruction iTU, second line current instruction iTV, and third line current instruction iTW.
Next, a power source current detector 9 detects two line currents of the three line currents output from the three-phase AC power source 1 and the remaining one line current is then obtained by taking the sum of the two detected line currents and inverting the sign thereof, and the obtained three line currents are output as the first line current measurement result iFU, second line current measurement result iFV, and third line current measurement result iFW. It should be noted here that this power source current detector 9 may be also constructed to detect the three line currents of the three-phase AC power source 1, outputting these as first line current measurement result iFU, second line current measurement result iFV and third line current measurement result iFW.
Next, a current controller 106 receives these first line current instruction iTU, second line current instruction iTV, third line current instruction iTW, and first line current measurement result iFU, second line current measurement result iFV, and third line current measurement result iFW to be compared, respectively, and generates a first switching instruction signal PU, second switching instruction signal PV, and third switching instruction signal PW controlling such that, the first line current instruction iTU and first line current measurement result iFU, the second line current instruction iTV and second line current measurement result iFV, and the third line current instruction iTW and third line current measurement result iFW are respectively coincident with each other as closely as possible.
Next, a main circuit power control section 8 includes the smoothing capacitor 60 and a switching power device group having a three-phase bridge construction. The switching power device group is comprised of a first switching power device Q1 connected to the plus terminal of the smoothing capacitor 60 for controlling the first line current IU, second switching power device Q2 connected to the plus terminal of the smoothing capacitor 60 for controlling the second line current IV, third switching power device Q3 connected to the plus terminal of the smoothing capacitor 60 for controlling the third line current IW, fourth switching power device Q4 connected to the minus terminal of the smoothing capacitor 60 for supplying the first line current IU to the three-phase AC power source 1, fifth switching power device Q5 connected to the minus terminal of the smoothing capacitor 60 for controlling the second line current IV, sixth switching power device Q6 connected to the minus terminal of the smoothing capacitor 60 for controlling the third line current IW, where each switching power device has a reflux diode connected in parallel thereto.
By this construction, any one of the first switching power device Q1 and fourth switching power device Q4 is turned ON in response to the first switching instruction signal PU, any one of the second switching power device Q2 and fifth switching power device Q5 is turned ON in response to the second switching instruction signal PV, and any one of the third switching power device Q3 and sixth switching power device Q6 is turned ON in response to the third switching instruction signal PW.
The description is now given assuming an arrangement such that, when the first switching instruction signal PU is L level, the first switching power device Q1 is turned ON, while when the first switching instruction signal PU is H level, the fourth switching power device Q4 is turned ON, and when the second switching instruction signal PV is L level, the second switching power device Q2 is turned ON, while when the second switching instruction signal PV is H level, the fifth switching power device Q5 is turned ON, and when the third switching instruction signal PW is L level, the third power switching device Q3 is turned ON, while when the third switching instruction signal PW is H level, the sixth switching power device Q6 is turned ON.
In the case where the voltage between the plus terminal and minus terminal of the smoothing capacitor 60 gets below the maximum value of the phase voltage of the three-phase AC power source 1, the three-phase AC voltage is rectified by the reflux diodes of the switching power device group Q1 to Q6.
FIG. 15 shows a conventional construction of the current controller 106 included in the conventional PWM converter system shown in FIG. 14. Also, FIGS. 16A to 16E show an operation of FIG. 15.
In the current controller 106, the first, second and third line current instructions iTU, iTV, iTW and the first, second and third line current measurement results iFU, iFV, iFW are respectively subtracted by means of subtraction units 117, 118 and 119 so as to obtain first, second and third line current error signals iEU, iEV and iEW. The first, second and third line current error signals iEU, iEV and iEW are input to first, second and third current error amplifiers 120, 121, 122 respectively to output first, second and third voltage instruction signals VU, VV and VW to be fed to a three-phase PWM signal generator section 139. For each of the current error amplifiers 120 to 122, a PI type (proportional/integral type) amplifier is typically employed as shown in FIG. 17, and its gain characteristic is obtained by the following equation: EQU G=R2.times.(R3.times.C1.times.S)/[R1.times.{(R2+R3).times.C1.times.S+1}]
The three-phase PWM signal generator section 139 includes first, second and third comparators 123, 124 and 125 and a triangular wave generator 126 generating a triangular wave signal SC to be input to minus terminals of the first, second and third comparators 123, 124 and 125. The first, second and third comparators 123, 124 and 125 receives the first, second and third voltage instruction signals W, VV and VW at their plus input terminals and compare the respective voltage instruction signals W, VV, and VW with the triangular wave signal SC, to thereby generate the first, second and third switching instruction signals PU, PV and PW.
In this construction, it is assumed that, when the voltage instruction signals W, VV and VW are respectively larger than the triangular wave signal SC, the first, second and third comparators 123, 124 and 125 generate H level, and in the meanwhile, when smaller, the comparators generate L level.
FIGS. 16A to 16E show the operation of the current controller 106 shown in FIG. 15, noting here that the operation is shown for the case where the first, second and third line current instructions iTU, iTV and iTW are three-phase sine waves.
In FIG. 15 and FIGS. 16A to 16E, in considering the gain of the current error amplifiers 120 to 122, it can be seen that, the larger the gain of the current error amplifiers, the nearer the line current instructions and line current measurement results approach each other and the line current errors can be made small, improving the response characteristic of the line current measurement results with respect to the line current instructions.
However, with the above conventional construction, due, for example, to phase lag produced by reactors 59, phase lag of the current error amplifiers, and dead time delay in the three-phase PWM signal generator section 139, if the gain of the current error amplifiers is made too large, oscillation phenomenon occurs. Therefore, normally chosen is a value of the current error amplifier gain as large as possible in a range such that oscillation does not occur. This gain value of each current error amplifier is determined at the design stage by studying the loop transfer function of the current control loop from the viewpoint of the characteristics of the three-phase AC power source 1, reactors 59, power source current detector 9, current controller 8 and main circuit power control section 8. In determination of the gain, it is necessary to lower the gain to a degree where oscillation does not occur even in the worst case, taking into account manufacturing variation of these characteristics and temperature characteristics. The task of determining this gain requires a lot of effort in design and on-site and requires considerable efforts in management in manufacture and on-site.
Also, since the optimum gain of the current error amplifiers changes depending on the DC voltage, a system must be constructed in which the gain is variable.
Next, there is a problem that, because the offset and/or drift of the triangular wave generator and the current error amplifier per se have an adverse effect on the current control error and/or restrict the range of dynamic drift, operational amplifiers are required such that the offset and drift of these components are small, and in some cases an offset adjustment operation is necessary during a manufacture process, which increases costs.
It should be noted that, although FIG. 15 is a conventional example in which the current controller 106 is implemented by analogue circuitry, the first, second and third line current measurement results iFU, iFV, iFW can be converted to digital data and the same constitution can be implemented by digital circuitry such as a microcomputer. Even in this case, the gain of the current error amplifiers must be determined by studying the loop transfer function of the current control loop in view of the characteristics of the three-phase AC power source, power source current detector, current controller and main circuit power control section, and this task is the same as in the case of implementation by analogue circuitry.
Furthermore, when the current error amplifiers are implemented using digital circuitry such as a microcomputer, since the offset and drift of the current error amplifiers themselves are digital calculations, they can be eliminated. However, phase lag gets larger as such calculation processing time increases and this tends to facilitate oscillation. This results in the problem that gain can not be increased without making the processing time very short, necessitating the use of a microcomputer or the like of very fast calculation processing capability, which is expensive undesirably.
Also regarding the A/D converter for converting the first, second and third line current measurement results iFU, iFV, iFW to digital data, phase lag increases as the conversion time gets longer, which facilitates to cause oscillation. This results in that the gain could not be increased without making the conversion time very short, necessitating the use of an A/D converter of very fast conversion capability, which is expensive. Also, offset and drift in the A/D conversion adversely affect the current control error and restrict the dynamic range, and therefore an A/D converter must be selected to have very small offset and drift, resulting in the problem of high cost.